1. Field of the Invention
The present invention relates to an objective lens unit and a recording/reproduction apparatus employing this objective lens unit for carrying out recording and reproduction of a data signal onto/from an optical recording medium such as an optical disk.
2. Description of the Prior Art
An optical recording medium such as an optical disk, magneto-optical disk and an optical card has been widely used for storing data such as dynamic image data, voice data, and computer data, because such an optical recording medium can be produced by mass production at reasonable costs. The rapid progress of our information-oriented society requires a higher recording density and a larger capacity of the optical recording medium.
In order to increase the recording density of a data signal in such an optical recording medium, it is effective to reduce the wavelength of the laser beam used to read out the data signal and to use an objective lens having an enhanced NA (numerical aperture) for focusing the laser beam on a magnetic disk. This is because the minimum size of a beam spot formed by focusing the laser beam cannot be reduced lower than .lambda./NA(.lambda.:wavelength of the beam) because of the diffraction.
As for the reduced wavelength of the laser beam, a blue laser diode and blue or green SHG laser are now under development. As for the enhanced NA of the objective lens, an objective lens used for a so-called "compact disk (CD)" (a digital optical disk for an audio signal or a computer) has a NA of 0.45, whereas an objective lens used for a so-called digital video disk (DVD) (digital optical disk for a video signal) having a higher recording density than the "compact disk" has a NA of 0.6.
It should be noted that in the digital video disk, with the purpose of reducing the effects of frame aberration caused by a disk inclination, the thickness of a transparent disk substrate is 0.6 mm which is half of the thickness 1.2 mm of a transparent disk substrate used in the "compact disk (CD)" and the magneto-optical disk.
In order to realize a data signal recording density than the aforementioned digital video disk, it is necessary to provide an objective lens unit having a NA exceeding 0.6.
The objective lens unit should be adjusted corresponding to the optical recording media of the respective specifications with different thicknesses of the transparent substrate and different NA values. Here the adjustment of the NA is especially important because the frame aberration caused by disk inclination is increased in proportion to the cube of the NA and the thickness of the transparent disk substrate.
In order to cope with the respective thicknesses of the transparent substrate in the aforementioned recording media of different specifications, it is necessary to prepare a plurality of lenses having different NA values, or to insert a diffraction-type hologram in the optical path. In these methods, the NA is defined in the stage of designing the lens or hologram corresponding to each of the specifications of the optical recording media.
The aforementioned objective lens unit is adjusted independently for each of the different specifications of the optical recording media. That is, it is necessary to prepare lenses for the respective thicknesses of the transparent substrate of the optical recording media and the respective NA values, or to employ the diffraction-type hologram.
In a case when preparing a plurality of lenses, it is necessary to exchange lenses each time when the aforementioned optical recording medium is changed. Moreover, it is difficult to design a compact objective lens unit having a plurality of lenses. On the other hand, there is also a problem to employ the aforementioned diffraction-type hologram in the aspects of costs and technology.